The present invention relates to a method for correcting a defect in a liquid crystal panel to correct a defective pixel existing in the liquid crystal panel.
A type of liquid crystal panel can be driven by an active matrix driving system. A liquid crystal panel of this type has first and second transparent substrates. The first transparent substrate has a transparent common electrode on a surface opposing the second transparent substrate, and the second transparent substrate has pixel electrodes arranged in a matrix on a surface opposing the first substrate. Each pixel electrode is connected to a switching element, such as a TFT (Thin Film Transistor). A liquid crystal layer of a twist nematic (TN) type is formed between the two transparent electrodes. The pixel electrodes can be selected or not selected by the switching operation of the respective switching elements, thereby displaying an image. In general, light-shielding black stripes are provided around each pixel electrode in order to increase the contrast of the image. Further, a polarization plate is provided on the outer side of each transparent substrate.
In the liquid crystal panel as described above, if pixels, wires or switching elements come to be short-circuited due to scatter of a pixel electrode, a wire or a foreign substance, the pixel including the short-circuited portion becomes defective. In a case of a liquid crystal panel of a normally white type in which a display screen is set white when no voltage is applied, the defective pixel including a short-circuit is recognized as a luminescent spot defect, which always transmits light. To correct this defective pixel, a laser beam is applied to the pixel to cut or scatter the conductive material, which causes the short circuit, thereby correcting the pixel to a normal pixel. Alternatively, a pixel electrode, wire member, aligning film, or foreign substance may be scattered to cause the pixel to display black. The reason why a defective pixel is corrected by the latter measure will be described. In the case of a liquid crystal panel of a normally white type, a voltage is not normally applied across a pixel electrode and a common electrode by a short circuit, and a defect appears on the panel as a white luminescent spot. Therefore, the defect is conspicuous and a reproduced image cannot have a high quality.
When a laser beam is applied to such a defective pixel, a pixel electrode, wire member, aligning film, or foreign substance is scattered and deposited on the aligning film. In this state, the liquid crystal is always oriented in the vertical or horizontal direction, i.e., not twisted. As a result, the pixel always displays black. If the defective pixel always displays black like a light-shielding black stripe, the adjacent pixels are influenced little, with the result that the display quality is improved.
The conventional method for correcting a defective pixel by radiating a laser beam will be described below. First, when a defective pixel is detected and irradiated with a laser beam, a bubble 4 is generated radially in a liquid crystal layer 3 sealed by a sealing member 2 between a pair of transparent substrates 1, as shown in FIGS. 9A and 9B. At the same time, the electrode, aligning film, foreign substance, etc., are evaporated or broken, and then scattered. When the conductive material causing the short circuit is cut by the laser beam, the defective pixel is corrected to show a high display quality. In the case of turning the pixel to always display black, after the pixel electrode, wire member, aligning film, or foreign substance is scattered and deposited on the aligning film and the bubble 4 is eliminated, the orientation of the liquid crystal molecules is turned from the TN type orientation to the vertical or horizontal orientation. As a result, transmittance of light is normally prohibited, i.e., the pixel displays black.
In the method for correcting a defective pixel described above, when a laser beam is applied to the defective pixel, the bubble 4 is generated, and a substance broken by the laser beam is scattered in a radial direction and deposited on the surface of the aligning film. The substance is scattered in a radial range shaping like a star, as shown in FIG. 9A; that is, some part of substance remains within the region of the defective pixel, another part extends to an adjacent pixel, and still another part extends to a farther pixel. Therefore, since the scattered substance is not deposited uniformly on the defective pixel, display in the pixel is not uniform, which adversely influences display in the adjacent pixels also. Thus, it is difficult to assure correction to a high display quality.